This invention relates to mechanically driven, infinitely variable gear ratio transmissions.
The need for a practical, infinitely variable gear ratio transmission has been recognized for years, since such a transmission permits selection of the gear ratio which results in the most energy efficient transmission of power from the source to the load.
The infinitely variable gear ratio transmissions of the prior art focus primarily on friction drive arrangements. For example, one type of friction drive transmission includes two conical pulleys, having parallel axes but opposite taper directions, connected by a belt, and mounted on an input shaft and an output shaft, respectively. The conical shape of the pulleys permits the gear ratio of the transmission to be changed by moving the belt in the direction of the conical axes. A modification of this transmission eliminates the need for a belt by providing conical wheels on the input and output shafts which frictionally engage an intermediate roller. The gear ratio of this transmission may be changed by changing the relative position of the wheels and the intermediate roller. Many other types of friction drive transmission have been disclosed, however, all have a common problem in that the friction drive is subject to slippage, particularly when high torque is applied. Thus, the friction drive transmissions are inherently inefficient and torque limited, and any benefits derived from infinitely variable gearing are largely offset by these inadequacies.
The prior art has endeavored to overcome the disadvantages of the friction drive by providing means for nonfrictionally connecting the input shaft to drive the output shaft. Such means, for example, may include one-way clutches mounted on the output shaft, which positively engage the output shaft when driven in one direction but overrun the shaft when driven in the opposite direction. These clutches may be driven by a plurality of cam followers which are oscillated by a rotating cam. Although these locking one-way clutches have some transmission losses associated with them due to play or backlash, such losses are small compared to the losses of dynamic friction drive transmissions, especially when high torque is applied. Thus, use of the one-way clutches significantly improves the efficiency of the transmission.
As previously mentioned, these clutches, and thus, the output shaft, are typically driven by a plurality of cam followers oscillated by a rotating cam. As the cam rotates, its eccentrics oscillate each of the followers sequentially, and therefore, only one follower supplies power to the output shaft at any given moment. However, the follower oscillations in these prior art transmissions typically occur at a sinusoidally varying velocity. Thus, since only one follower drives the output shaft at a time, their sinusoidally varying velocity will be transmitted to the output shaft. This creates undulations in the output shaft velocity, thereby causing the output to be rough. The prior art discloses that the output can be smoothed to some extent by adding more followers, however, this increases the complexity and cost of the transmission.
In addition, since the rotation of each follower varies essentially sinusoidally, the torque capability of the transmission is limited to that which can be handled by a single follower/clutch combination.
The gear ratio of this type of prior art transmission is determined by the magnitude of oscillation of the follower arms relative to the amount of cam rotation. The magnitude of follower arm oscillation may be increased by moving the rotational axis of the cam and the rotational axis of the follower arm closer together. However, the rotational axis of the cam is coincident with its physical axis, and the rotational axis of the follower arm is typically coincident with the physical axis of the shaft which supports it. Therefore, the proximity of the two rotational axes is physically constrained, since the cam and follower rotational axes cannot be so close together that the cam will strike the shaft supporting the follower arm. This fact limits the applications of such transmissions to those which allow a significant gear reduction within the transmission itself. While such reduction can be overcome, in some applications, by providing a correcting, non-variable transmission in the drive line, this is accomplished only at the expense of decreased efficiency and increased cost, i.e., greater transmission complexity.